Flat-chested kitten syndrome (FCKS), is a disorder in cats, wherein kittens develop a compression of the thorax (chest/ribcage) caused by lung collapse. This is a soft-tissue problem and is not caused by vertebral or bony malformation. However lung collapse can be a secondary symptom caused by bony deformity affecting the thorax such as pectus excavatum. In mild cases, the underside of the chest becomes flattened (hence the name of the condition); in extreme cases the entire thorax is flattened, looking as if the kitten has been stepped on. The kitten will appear to go from normal to flat in the space of about 2–3 hours, and will usually then stabilise.

FCKS is most frequently caused by collapsed lungs (and not as formerly believed, by a muscle spasm in the intercostal muscles). There are numerous causes for lung collapse, and therefore numerous causes for FCKS. One possible cause for flat chestedness that develops soon after birth is atelectasis.

Causes of atelectasis include insufficient attempts at respiration by the newborn, bronchial obstruction, or absence of surfactant (a substance secreted by alveoli that coats the lungs and prevents the surfaces from sticking together). Lack of surfactant reduces the surface area available for effective gas exchange causing lung collapse if severe. There can be many reasons for atelectasis in kittens, but probably the commonest cause is prematurity. Newborn atelectasis would not be unusual in a very large litter of kittens (such as 10), where the size of the litter may lead all the kittens to be small and mildly underdeveloped.

Unlike human babies, kittens are born very immature: blind, deaf, the intestinal tract not fully developed etc., so even slight prematurity may tip them over the edge from being viable to non viable. Many FCKS kittens may have fallen just the wrong side of this boundary in their development at the time of birth. Further, if a kitten does not scream or open its lungs well enough at birth, even if it is fully mature and has sufficient surfactant, it may end up with atelectasis. Patches of atelectasis in the lungs mean that part of a lung is not operating properly. If the kitten goes to sleep and its respiratory rate drops, the patches of atelectasis can slowly expand until large areas of the lung collapse and cannot be reinflated. Good advice to any breeder therefore would be to ensure that kittens cry loudly when they are born, to make sure that the airways are clear, but also that the lungs expand as fully as possible. (This was the reason newborn babies were always held upside down immediately after birth (so that any residual fluid drains downwards) and smacked to make them cry strongly.)

Some kittens suffer from congenital 'secondary' atelectasis, which presents shortly after birth. There have been no reports of kittens born flat (primary atelectasis). Hyaline membrane disease is a type of respiratory distress syndrome of the newborn in which there is formation of a hyaline-like membrane lining the terminal respiratory passages, and this may also be a (rarer) cause of FCKS. Pressure from outside the lung from fluid or air can cause atelectasis as well as obstruction of lung air passages by mucus resulting from various infections and lung diseases – which may explain the development of FCKS in older kittens (e.g. 10 days old) who are not strong enough to breathe through even a light mucus, or who may have inhaled milk during suckling.

Tumors and inhaled objects (possible if bedding contains loose fluff) can also cause obstruction or irritation of the airway, leading to lung collapse and secondary atelectasis. In an older cat the intercostal muscles are so well developed, and the ribs rigid enough that the ribcage will not flatten if the lung collapses: in kittens the bones are much more flexible, and the tendons and muscles more flaccid, allowing movement of the thorax into abnormal positions.

Other causes of lung collapse can include diaphragmatic hernia, or diaphragmatic spasm (breeders report the position of the gut and thorax as appearing to be like a 'stalled hiccup'). Diaphragmatic spasm is easily checked by pinching the phrenic nerve in the neck between the fingertips. Kittens with this type of FCKS will improve almost immediately, but may require repeated pinching to prevent the spasm from recurring.

FCKS develops usually in kittens around three days of age, and sometimes affects whole litters, sometimes only individuals or part of a litter. Kittens can go flat any time during early maturation, some flattening as late as 10 days of age or (in very rare cases) later. It is possible that the later-developing cases are due to Respiratory tract infection or pneumonia. Until 2010 FCKS was believed to be caused by a spasm in the intercostal muscles, but new data has led to the conclusion that flattening is caused by failure of the lungs to inflate normally or, when it occurs in older kittens, by lung collapse.

Gross physical symptoms include flattening of the underside of the thorax in moderate cases (a ridge can usually be felt along the sides of the ribcage, running parallel to the spine); complete flattening of the upper body in extreme cases (the kitten looks as if it has been stepped on); moderate to extreme effort and/or gasping during breathing; the gut is drawn upwards during the in-breath. The position of the thorax and activity of the abdomen is not unlike that seen during normal hiccups, but the sudden spasm in hiccups is slowed down or halted in FCKS: where a hiccup releases, returning the body to a normal position, FCKS breathing does not release. There may be involvement with digestive difficulty in FCKS kittens (see Colic, below).

Banana used to demonstrate FCKS shape

Determining whether a kitten has FCKS or not can be difficult with only text descriptions: a mild case of FCKS causes the thorax to feel similar to the shape of a banana when held curve downward. The ribcage is not fixed in position, and the most noticeable effect in mild cases is the ridge along the side of the ribcage.

The condition causes weight-gain to halt, respiratory distress, inability to feed normally and, in a significant proportion of cases, death. However, since a significant percentage of kittens survive the condition immediate euthanasia is not indicated, and supportive treatments can be employed to increase the likelihood of survival (see Treatment, below).

The condition is often misdiagnosed as Pectus excavatum,[1] with which it has no direct connection, although FCKS kittens may also have PE. Although the condition is believed to be more prevalent in the Burmese breed it is found in every breed of cat, including non-pedigree domestic cats, and the apparent prevalence in the Burmese is most likely due to better communication between breeders and reporting of the condition, as well as the naturally more barrel-shaped chest of this particular genotype.[2] Since early reporting of the condition identified the Burmese as susceptible the Bengal breed, with a similar physiology, has emerged, and shows a similarly large number of FCKS kittens, however this may be due to specific interest in the condition among those working with the breeds. It is reported in all breeds and in domestic non-pedigree cats, both those kept as pets and those living as 'barn cats'. An article in a Swedish cat club newsletter about FCKS led to a spike in reporting of the condition in Ragdolls in Sweden.

Egyptian Mau kitten with FCKS

Rex kitten with FCKS and Kyphosis - note narrowing of thorax around the shoulders

The syndrome is life-threatening in a significant number of cases (possibly around 50-60%) mainly due to a lack of understanding of the underlying cause of the condition, failure to treat colic (leading to slow starvation) and insufficient sources of information in veterinary literature.

In some cases (particularly where whole litters are affected) FCKS can be due to genetic factors: certain bloodlines are known to produce a preponderance of kittens with the condition, and close matings very commonly produce it in the offspring. Isolated cases are more likely due to environmental factors or slight prematurity of the kittens concerned, and even complete litters suffering from FCKS may have no genetic component—repeat matings that have produced flat kittens in one instance have often not produced it again, and recovered FCKS that have been bred from have likewise not necessarily produced offspring that suffered from the condition.

It is possible that the lung collapse in neo-nates (i.e. around 3 days of age) could be due to a lack of surfactant - the coating of the inside of the lung that prevents the inner surfaces of the alveoli from sticking together. (Causes of surfactant deficiency are not discussed here, but the role of surfactant is discussed in a number of articles in the British Journal of Anaesthesia vol. 65, 1990.[3]). Kittens are born immature in many ways, and full inflation of the lungs does not happen immediately, but takes place gradually over a period of several days after birth. Although a kitten may seem active and thriving, its lungs will not be fully inflated until approx. day 3 after birth. Thus if inflation fails to happen correctly the negative pressure in the chest cavity will cause the ribcage – which is extremely flexible – to collapse inward, dragged in by the lungs, and not collapsing due to muscular cramping, and compressing the lungs.

Neo-natal and Later lung collapse may be due to lung infection, or possibly to a malfunction in the epiglottis, causing the in-breath to draw air into the digestive tract rather than the lungs. A short term malfunction of this sort may be perpetuated by the resulting colic creating a feedback loop that interrupts the correct breathing process.

Autopsy and analysis of lung aspirate in a group of flat-chested kittens bred by a US Vet showed the presence of Herpes virus.

Spasm in the diaphragm leads to the muscle 'locking up' so that all breathing effort falls to the intercostal muscles. The resulting loss of movement causes the lungs to deflate gradually. This is easily diagnosed and treated (see Treatment below) by short-term interruption of the Phrenic nerve.

A kitten that has difficulty in breathing is very likely also to suffer from colic (which can cause weight loss in the early development of a normal kitten), and a very small daily (or twice daily) dose of liquid paraffin (one or two drops placed on the tongue of the kitten, or 0.1 ml) should help to alleviate this problem. FCKS kittens who do not maintain weight are usually among the group which die, but many of them may simply be unable to feed properly due to colic, becoming increasingly weak and lethargic, and fading due to malnutrition as much as to the thoracic problems.

Colic has many causes, but in a kitten with respiratory difficulty it is possible that a malfunction during the breathing process leads the kitten to swallow air instead of taking it into its lungs. The GI tract fills with air while the lungs do not receive a proper air supply, preventing them from inflating fully. Pressure from the stomach exacerbates the condition. Treating for colic with liquid paraffin seems to shorten recovery time from 4–10 weeks to a matter of days.

Position of Phrenic nerve in a sleeping kitten. The nerve bifurcates and runs down both sides of the neck.

Treatment is difficult to define given the number of different causes and the wealth of anecdotal information collected by and from cat breeders. Treatments have hitherto been based on the assumption that FCKS is caused by a muscular spasm, and their effectiveness is impossible to assess because some kittens will recover spontaneously without intervention.

Continuous positive air pressure (CPAP) is used in human babies with lung collapse, but this is impossible with kittens. It is possible that the success of some breeders in curing kittens by splinting the body, thus putting pressure on the ribcage, was successful as it has created the effect of positive air pressure, thus gradually re-inflating the lungs by pulling them open rather than pushing them open as is the case with CPAP.

The use of steroids (Dexamethasone) coupled with an antibiotic (Amoxicillin) will support the kitten in a number of ways, the steroid enhancing maturation and the antibiotic addressing the possibility of underlying infection and compensating for the immuno-depressant properties of the steroid. The steroid will also encourage the kitten to feed more energetically, keeping its weight up. Several breeders believe that Taurine plays a part in the condition, and it may be that some cases are Taurine-related. These breeders give the queen large doses of Taurine (1000 mg) daily until the kittens recover – apparently within a few days.[4] Given that most FCKS cases take weeks rather than days to recover, this supplement may be relevant.

Splinting the kitten in a specially-constructed corset encourages the ribcage to a more normal position, and reported mortality seemed to decline when this practice was introduced.[5] This may be because encouraging the chest to a more correct position helps the lungs to re-inflate. However a large proportion of kittens cannot tolerate a splint, and the distress it causes is extremely counterproductive. It can also be dangerous in cases where pressure on the sides causes the sternum to move inwards rather than outwards, and should only be undertaken with veterinary support and advice. Some kittens recover without intervention, so it is not known whether the various treatments based on encouraging the thorax to return to a normal shape contributed to recovery.

Some breeders have found that putting pressure on the sides of the ribcage can help, as the chest rounds out with encouragement (gentle pressure timed to coincide with the natural movement of the thorax in breathing), but usually the chest goes flat again as soon as massage is discontinued. It may however help in encouraging the lungs to inflate a little more with each breath, but should not be used if it causes distress to the kitten. Many breeders report that affected kittens seem to enjoy massage. Encouraging a kitten to lie on its side can be helpful, and draping another kitten (or the mother's arm) over it while it is sleeping places a gentle continuous pressure on the ribcage which may also be helpful. If the kitten is uncomfortable it will move away. (It is important to ensure that pressure is not placed on the kitten if it is lying flat on its chest.)

Over-handling FCKS kittens can lead to unnecessary weight loss and lethargy, so the use of massage, waking the kitten for extra feeds from the mother etc. should be checked against gains and losses in weight. Some vets believe that encouraging the kitten to cry and also to have to move more to reach its mother may be helpful. There is little or no data from breeders supporting this theory.

It is difficult to determine whether a kitten that goes flat will survive or not. A good indicator is the weight of the kitten: those that continue to gain weight generally have a better chance of survival. Supplement feeding is therefore recommended in all cases, together with vitamin supplements,[6] although many of these kittens will not accept hand feeding. Liquid Paraffin to alleviate colic seems to be significant in assisting normal feeding and weight-gain.

Another indicator to the severity of the case is the use of the stomach when breathing: normal kittens use only the ribcage, a flat-chested kitten may manage to breathe only using the ribcage, or may suck the gut upwards with every breath – if the latter is the case then the likelihood of survival seems to be lower, though still not sufficient to warrant immediate euthanasia. If the condition is stable (i.e. the flatness does not increase over time) or improving, the kitten is more likely to survive. If the condition worsens over several days, survival is less likely.

Kittens with FCKS may die (or have to be euthanased) very soon after onset. There are two points at which breeders report kittens that were otherwise doing well deteriorating and dying: at 10 days of age and at 3 weeks. Generally if the kitten is still flat, but survives the 3-week developmental stage, its prognosis is good. Many will have returned to a normal shape by this time. Those retaining some degree of flatness often grow out of the condition at any point in the ensuing 6 months, and the vast majority of survivors appear to lead normal lives with no side-effects, either physical or immunological.

FCKS kittens that survive but who have not been given any drug treatment or support other than supplementary feeding, generally recover over a period of 4–10 weeks, and are usually normal by 12 weeks of age, though some take as long as 6 months to normalise. In the very small number of kittens reported so far treated with steroids, antibiotics and liquid paraffin (to address colic) recovery is usually seen within a matter of days. Given the number of different types of FCKS these kittens (all with the minor form of the condition) may not be representative of all cases. More data is required for statistical analysis.[7]

A small proportion of severe FCKS kittens are left with long-term respiratory problems, kyphosis, and in some cases cardiac issues caused by the compression of the thorax during the early developmental stages (particularly where the condition has been coupled with Pectus Excavatum). Cardiac issues are generally audible on physical examination; further indications include the kitten becoming breathless after play, less active than siblings, and failure to grow and develop normally.

Crucial in the decision to breed would be the primary cause of FCKS in the litter, which may or may not be genetic. Some recovered FCKS adults have produced FCKS offspring in their turn (or lines that consistently produce flat kittens), and breeding from them is therefore inadvisable. However, repeat matings in which FCKS has appeared does not always result in further FCKS kittens. Queens and studs who consistently throw complete litters of kittens with the condition are generally neutered since a genetically linked cause for the condition can be introduced into lines that do not produce it by breeding with lines in which it is common. Isolated instances of single flat kittens in an otherwise healthy litter are unlikely to have a genetic component in the condition, and neutering of parents of such kittens is not usually necessary in pedigree breeding, especially since this may have detrimental effects on the gene pool.

If the cause of flattening is colic related to over-production of milk then this would not be cause for neutering. The only way to determine if the cause is digestive would be if the condition was alleviated in all cases by pinching the phrenic nerve and/or use of liquid paraffin to relieve colic resulting in improvement in the condition.

Line-breeding or inbreeding is highly inadvisable in lines where FCKS has appeared, and the practice may cause the condition to appear in lines where it has not previously been recorded.

Kittens with FCKS sometimes also show bony deformities such as Pectus Excavatum or Kyphosis (characterised in kittens by a dip in the spine just behind the shoulder blades). Although a kitten may be born with these deformities, extreme forms of FCKS may cause Kyphosis to develop in an otherwise normal kitten, as the kitten grows with the spine held in an abnormal position due to the flattened ribcage. When there are bony deformities in the upper body, or other problems present, these may contribute to malfunctioning in soft-tissue organs such as the respiratory and digestive systems.

Flat chest occurs in piglets and puppies, may be known in cattle (anecdotal information only), and is also recorded in humans, though possibly in humans this is not the same type of condition and may be a bone deformity.